The broad objective of the project is to study growth characteristics of rickettsiae in cell cultures, both monolayers and suspended cells. Data obtained are applied to differentiation and classification of rickettsiae and to routine production of them for use in serologic and immunologic studies in collaboration with other investigators. Major emphasis thus far has been placed on members of the spotted fever group and certain strains which are presently unclassified, particularly those which grow poorly in yolk sacs of embryonated eggs. Standard tissue culture procedures are used except that inocula are centrifuged onto cell monolayers when doing so greatly enhances infection. Different host cells and environmental conditions are tested in order to find the combination which provides the most prolific rickettsial growth. Mouse L and monkey Vero cells have been particularly useful, being relatively easy to cultivate and susceptible, one or both, to most strains of rickettsiae studied. The project also includes preparation of purified and semipurified concentrates of cell culture-grown rickettsiae for use as antigen, together with development of methods for such purification.